The use of interference screws to fix the ends of a bone-tendon-bone complex to the walls of bone tunnels is well known in the art. It is generally accepted that an interference screw provides sufficient fixation of a bone block in a bone tunnel to allow for early rehabilitation after ligament surgery, such as in anterior cruciate ligament (ACL) reconstruction surgery.
Some techniques avoid using a bone block to fix a ligament in a bone tunnel. Referring to FIG. 1, one such technique involves suspending a ligament L in a bone tunnel C with a pull-through suture S. Over time, the ligament L and surrounding bone grow together, fixing the ligament L in place.
Note: As used herein, the term “ligament L” is meant to refer to the replacement ligament being fixed in a bone tunnel. To that end, the replacement ligament L might comprise:                1. ligament or tendon or other tissue harvested from the patient's own tissue (known as autograft tissue); or        2. ligament or tendon or other tissue harvested from another human donor (known as allograft tissue); or        3. ligament or tendon or other tissue harvested from an animal source (know as xenograft tissue); or        4. ligament or tendon or other tissue made by tissue engineering; or        5. prosthetic ligament made of artificial material; or        6. a combination of the above.        
Referring to FIG. 2, another technique uses an interference screw to directly fix ligaments without a bone block (such as a semitendonosis and gracilis tendons) to the walls of the bone tunnel. In this case, the ligament L may be towed into place with suture S, which normally is removed after interference screw fixation has been achieved. However, one disadvantage of direct fixation of the ligament in a bone tunnel with an interference screw is the risk of slippage of the ligament under load prior to the ligament healing to the bone. Such slippage may occur during the rehabilitative period when the ligament is subjected to repetitive loading. To provide for a sufficient interference fit, a large interference screw may be required. However, these large interference screws may cause necrosis of the tendon because of excessive compression of the ligament against the bony tunnel wall.